s = 0
unseen = 1
seen(v) = bittest(s, v)
see(v) = s = bitor(s, 2^v); while (seen(unseen), unseen++)

base = 10;

other(p) = {
	see(p);
	my (v = oo);
	while (p,
		forstep (w = p, oo, base^#digits(p, base),
			if (!seen(w),
				v = min(v, w);
				break;
			);
		);
		p \= base;
	);
	return (v);
}

{
	a = vector(10 000);
	u = 1;
	for (n=1, oo,
		v = if (n==1, 1, other(v));
		if (v <= #a,
			a[v] = n;
			while (a[u],
				print (u " " a[u]);
				if (u++ > #a,
					break (2);
				);
			);
		);
	);
}

quit